Demand for secondary batteries as an energy source is rapidly increasing due to the increase in technological development and demand for mobile devices. Among such secondary batteries, lithium secondary batteries, which have a high energy density and voltage, a long lifetime, and a low self discharge rate, have been commercialized and are widely used.
However, lithium secondary batteries have a limitation in which repeated charge/discharge cycles dramatically reduce the lifetime of the batteries. In particular, the limitation is more severe in long-lifetime or high-voltage batteries. This is due to effects which are generated when moisture inside the batteries and the like causes electrolyte decomposition or active material degradation, as well as an increase in the internal resistance of the battery. In particular, in the case of positive electrode materials, when deterioration of the positive electrode material itself increases in severity, the elution of constituent elements from the positive electrode active material increases, and consequently, the lifetime of the battery decreases sharply or the battery becomes unusable at high voltages.
In order to overcome such limitations, methods for forming a surface treatment layer on the surface of the positive electrode active material have been proposed. Among these, aluminum-based surface treatment layers, which are recognized for stability under high voltage and in electrolyte solutions, are coated—in a crystalline state—on the surfaces of particles, and are thus difficult to apply uniformly over the entirety of the active material Moreover, there is a limitation of increased resistance due to the crystallinity of the aluminum-based compound itself. Boron (B)-based coatings are uniformly coated in an amorphous state and thus do not hinder the movement of lithium ions from a positive electrode material to an electrolyte solution. However, boron-based coatings react with moisture, and thus have a limitation in which the boron-based coating is unable to function as a coating layer if the reaction is extended for a long period of time.
Accordingly, there is a critical demand for the development of a positive electrode active material that can overcome such limitations while improving lithium secondary battery performance.